Thermoplastic adhesive preform for heat sink attachment

ABSTRACT

Curved surfaces of a preform of thermoplastic adhesive provide improved regulation of heating and exclusion of gas as surfaces to be bonded are heated and pressed against the thermoplastic adhesive preform. Particulate or filamentary materials can be added to the thermoplastic adhesive for adjustment of coefficient of thermal expansion or further increase of heat transfer through the adhesive or both. The preform is preferably fabricated by molding, preferably in combination with die-cutting of a preform of desired volume from a web of approximately the same thickness as the completed bond.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to bonding of heat sinks tointegrated circuit packages and, more particularly, to improving heattransfer through adhesive bonds as well as improving integrity androbustness of the adhesive bonds.

2. Description of the Prior Art

Heat dissipation is a major factor in the design of semiconductordevices such as analog and power transistors and especially in highperformance digital switching circuits formed at high integrationdensity. Ideally, in switching devices, heating is produced only duringthe switching transition interval and a small resistance in the “on”state. However, in high-performance circuits which provide very rapidswitching transitions, it is generally the practice to exploit rapidswitching by increasing the clock frequency so that the switchingtransition occupies a relatively constant portion of a clock period.Therefore, heat dissipation requirements generally increase with clockspeed for each switching transistor.

Further, to exploit this increased clock frequency and to obtain reducedsusceptibility to noise (as well as reduce manufacturing costs) there issubstantial incentive to fabricate high-performance switching elementsat the maximum possible integration density to minimize the length ofsignal paths therebetween. Therefore heat dissipation requirements alsoincrease proportionally with integration density.

Accordingly, it has recently become the practice to incorporateattachment of a heat sink or other heat removal structure (e.g. aliquid-cooled cold plate) into the design and manufacture of integratedcircuit packages since heat removal is critical to both performance andreliability of the integrated circuit. In this regard, incorporation ofthe heat sink with the fabrication of the package is justified by thecriticality of heat transfer from the package to the heat sink sinceuneven cooling may cause stresses within the chip or between the packageand the heat sink. Thermal cycling can then damage the circuit elements(e.g. diffusion or oxide growth) or connections formed on the chip (e.g.metal fatigue or migration) or degrade the attachment of the heat sinkto the package which will then tend to increase the temperatureexcursion during thermal cycling.

For attachment of heat sinks to integrated circuit packages, it has beenthe practice to use an adhesive which has a relatively good thermalconductivity. However, the thermal conductivity of such materials isstill very low compared to metals. For example, the thermal conductivityof a thermally conductive adhesive in current use is only about 1.73W/m-° C. whereas copper has a thermal conductivity of 395 W/m-° C.Additionally, the interfaces of the package to the adhesive and theadhesive to the heat sink further impede heat transfer. Therefore, itcan be understood that the adhesive connection of the heat sink iscritical to both the thermal and electrical performance of thecombination of chip, package and heat sink.

Specifically, the cross-section of the thermal path must be maximizedand should not be compromised by gas or air bubbles. Such bubblespresent a region of reduced thermal conductivity and two additionalinterfaces to impede heat flow. Further, thermal cycling causesexpansion of the gas or increase of pressure within the bubble which cancause progressive breakage of the adhesive bond.

Additionally, it is known that a certain volume of adhesive is necessaryto provide sufficient robustness of the bond to resist damage thereto byroutine handling before or after the package is placed in service andthe same applies to damage from gas or air bubbles, as well. On theother hand, since the thermally conductive adhesive has a significantthermal resistance, the length of the thermal path through the bondshould be no more that required by the volume of adhesive necessary to arobust bond. Therefore, the thickness of the adhesive bond is relativelycritical to the thermal performance of the integrated circuit package aswell as structural integrity of the bond itself.

It has therefore been the practice to bond heat sinks to integratedcircuit packages with a reworkable thermoplastic adhesive which isinitially in the form of a sheet of a thickness designed to provide theproper volume and thickness of the bond. In this sense, the sheet isessentially an adhesive preform and presents the problems of arequirement for heating the entire assembly to form the bond andcapturing air or gas at the surfaces of the sheet while the assembly ispressed together for heating and bonding. Throughput is low due to thethermal mass which must be heated and cooled.

A dispensable adhesive is an alternative to an adhesive preform.Unfortunately, dispensable adhesives do not fully solve the problems ofa preform and present others. While air or gas will not generally betrapped by a dispensable adhesive that can flow when parts are pressedtogether, the thickness of the adhesive bond cannot be well-regulated.Further, good handling characteristics of dispensable adhesives such asease of dispensing, long storage and pot life and short cure timegenerally imply poor thermal performance and vice-versa. Poor thermalcharacteristics increase the criticality of the adhesive bond thickness.Epoxies with suitable thermal conductivity, after mixing, must stayfrozen until use, require special dispensing equipment, have a shortworking life and require a long oven cure. Suitable cyanoacrylateadhesives also require special dispensing equipment, the addition of anactivator for curing and require only light handling, at most, forseveral hours after the bond is made. Either the long oven cure requiredby the epoxy or the period of restriction on handling of the devicecauses a restriction on the duration of the manufacturing process.

Accordingly, it can be seen that known alternatives for bonding heatsinks to circuit packages all present some unavoidable complexity in themanufacturing process and the possibility of compromising manufacturingyield or reliability.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide athermoplastic adhesive preform which avoids entrapment of air or gaseswhile providing precise control of adhesive volume and bond thickness.

It is another object of the invention to provide enhancement of heatconduction through an adhesive bond while improving accuracy andrepeatability of bond thickness.

It is a further object of the invention to provide for use of anadhesive which does not require dispensing, is reworkable without damageto an electronic circuit package and may be conveniently handled andstored prior to use for bonding heat sinks to electronic circuitpackages.

In order to accomplish these and other objects of the invention, amethod of attaching a heat removal structure to an electronic circuitpackage is provided including the steps of forming a curved surface on apreform of thermoplastic adhesive, and heating and compressing thepreform between the electronic circuit package and the heat removalstructure to form an adhesive bond having a desired thickness.

In accordance with another aspect of the invention, a method for makinga preform of thermoplastic adhesive having a curved surface is providedincluding the steps of placing a desired volume of thermoplasticadhesive in a mold, and applying heat and pressure to said volume ofthermoplastic adhesive until it conforms to a curved inner surface ofsaid mold.

In accordance with a further aspect of the invention, an adhesivepreform of thermoplastic material is provided including at least onecurved major surface for contacting a surface to be bonded over aprogressively larger area as a bond of desired final thickness isformed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a cross-sectional view of a preferred form of a thermoplasticadhesive preform in accordance with the invention prior to bonding,

FIG. 1A is a plan view of the preform of FIG. 1,

FIG. 2 is a cross-sectional view of a completed bond between anelectronic circuit package and heat sink using the preform of FIG. 1,and

FIG. 3 is a cross-sectional view of a preferred method of forming thethermoplastic preform of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there isshown, in cross-sectional view, an adhesive preform 30 in accordancewith the invention together with an electronic package 20 and a heatsink 40 which are to be bonded thereby. As alluded to above, when a bondis to be formed, the electronic package 20 and the heat sink 40 areassembled together with the thermoplastic preform 30 sandwichedtherebetween and the package and heat sink pressed toward each other(e.g. by support 10 and press 50) while the entire assembly is heated.Heating of the entire assembly can be reduced somewhat by preheating ofthe adhesive preform prior to assembly.

It can be readily appreciated that if the adhesive preform issubstantially flat, irregularities in the surface of the package 20,preform 30 and/or heat sink 40 will prevent intimate contact over theentire interfaces between these components prior to bonding. Thus whenthe surfaces are pressed together, ambient gases will be trappedresulting in compromise of both heat transfer and the integrity androbustness of the bond. At he same time, heat transfer from the packageand heat sink to the adhesive preform is irregular prior to bonding dueto lack of intimate contact to the preform. Further, it should beappreciated that during heating to form the bond while the assembly isunder compression, the adhesive will flow somewhat and can extrude fromthe edges of the bond. Therefore, a flat preform does not provideregulation of the amount of adhesive in the bond or the thickness of thebond, particularly where gas may be trapped between the package and orheat sink and the preform.

The same is especially true of dispensed adhesives where adhesive may bedistributed in lines or dots over either or both of the surfaces to bejoined. Such discontinuities in the pattern of adhesive, as applied, isparticularly subject to the trapping of ambient gases as the adhesive iscompressed and flows between the surfaces to be bonded. Trapped gas willthus cause extrusion of more than the intended amount of adhesive if theintended thickness of the bond is obtained, resulting in less than theintended amount of adhesive remaining in the bond, or, alternatively,the thickness of the bond will be increased by an amount correspondingto the volume of the trapped gas. Both of these effects or any degree ofcombination of the two compromises both heat transfer and bondintegrity.

To provide for avoidance of the trapping of gases in accordance with theinvention, preform 30 is formed with a slight degree of curvature in thetop and bottom surfaces thereof so that the preform is of a convex orpillow-like shape in cross-section. It is preferred that the amount ofcurvature be chosen such that the initial maximum thickness t₀ of thepreform be at least 1.5 times the thickness of the final designthickness, t, (shown in FIG. 2) of the bond. Alternatively, thecurvature should be chosen in accordance with the dimensions of surfaceirregularities of the preform to avoid closing of any concave depressionin the surface of the preform before gases can be excluded therefrom bydeformation and flow of the preform such that the convex surfacecontacts a progressively larger area of a surface to be bonded as a bondof desired design thickness is formed. It should also be appreciatedthat the curvature provides an intimate contact between the central areaof the package 20 and/or heat sink 40 with the central portion of thepreform 30 so that heating of the preform will be both rapid andprogressive, aiding the exclusion of gases as the preform material isprogressively allowed to flow. In a limiting case, the preform 30 couldbe in the shape of a sphere of appropriate volume to accommodaterelatively extreme surface roughness of the preform if such surfaceroughness was not otherwise avoidable.

It is also considered desirable in view of the viscous flow of theadhesive under compression and at a suitably elevated temperature thatthe perimeter of the preform 30 be slightly smaller than the dimensionsof surfaces to be bonded, as indicated at 25 of FIG. 1A, and slightlyconcave in plan view so that the final shape of the preform 30′ willconform closely to that (25) of the areas to be bonded. Thus, since thevolume and shape of the adhesive can be controlled and trapping of gasescan be avoided, the onset of extrusion of adhesive 30′ from between thesurfaces to be bonded can be detected used to regulate the finalthickness, t, of the bond, as shown in FIG. 2, by terminating heatingand compression and the cross-sectional area of the heat transfer pathwill be similarly well-regulated.

As a perfecting feature of the invention, possible compromise of therobustness of the bond will be reduced as the coefficient of thermalexpansion of the adhesive is matched (e.g. by choice of thermoplasticadhesive material or the use of additional fill material 61 in aparticulate or powder form) to that of the heat sink and/or electronicpackage. However, the closeness of the match is not at all critical tothe practice of the invention. As a further perfecting feature of theinvention, particulate or filamentary form material 62 of high thermalconductivity can be added to the adhesive of the preform to increasethermal conductivity of the adhesive.

Referring now to FIG. 3, methods for forming the preforms of FIG. 1 willbe discussed. A preferred technique is to simply die cut andsimultaneously form the preforms 30 from a sheet or continuous web 80,which may have particulate or filamentary additives 61, 62 embeddedtherein, using a punch 70 and die 72 which have a shaped concaveinterior. The web 80 can then advantageously be of approximately thesame thickness as the intended thickness, t, of the completed bond.

The initial die cutting of the sheet or web 80 by punch blade 71 againstdie 72 establishes the amount of thermoplastic adhesive to form the bondand, as the punch 70 and die 72 are driven further together, thethermoplastic material, heated by heaters 74 and/or 75 is pinched at theedges and forced to flow toward the center of the preform or mold toform a pillow-like shape with curved surfaces. Gases are allowed toescape from the shaped interior of the punch 70 and die 72 through vents76. Thus, the preform is substantially molded to the desired shape andany residual stress in the preform will assist in causing adhesive flowin the preform which assists in achieving the desired shape of thecompleted bond (which will be substantially the same as that whenoriginally cut from sheet or web 80).

It should be understood that FIG. 3 can also be understood as beingillustrative of injection molding and other molding processes with theexception that in the latter case, a cutting edge 71 need not beprovided. The mold can also be charged with a preform of adhesivematerial of appropriate volume in an arbitrary shape (such as a sphereor a pellet of, for example, ellipsoid or cylindrical shape) for moldingto the preferred pillow shape with curved surfaces.

It should be noted from the foregoing, that the use of a thermoplasticmaterial allows the adhesive bond to be softened by the application ofheat so that a heat sink can be removed from an electronic circuitpackage without damage to the package. The bonding material can also beeasily removed from both the heat sink and the electronic package and,moreover, may be recovered and reformed by molding in the mannerdescribed above for reuse.

In view of the foregoing, it is seen that the invention provides forreliably and repeatably avoiding the entrapment of gases while forming areworkable bond of a heat removal structure such as a heat sink or coldplate to an electronic circuit package. The thermoplastic adhesivepreforms also provide for accuracy of bond volume and thickness, theavoidance of dispensing and ease of handling and storage of the adhesiveprior to use. Design dimensions of the bond to optimize bond integrityand robustness are accurately realized to optimize heat transfer whileallowing a portion of the volume and area of the bond to be allocated tomaterials for increasing heat transfer capacity across the adhesive bondand/or adjustment of the coefficient of thermal expansion of theadhesive.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1-20 (Cancelled).
 21. A method for making a preform of thermoplasticadhesive having a curved surface, said method including the steps of:placing a desired volume of thermoplastic adhesive in a mold havingopposing concave inner surfaces; and applying heat and pressure to saidvolume of thermoplastic adhesive until it conforms to the concave innersurfaces of said mold, thereby forming a thermoplastic preform havingopposing convex outer surfaces.